Several prior art patents disclose devices for dispensing small quantities of liquids. Most rely on capillary action to dispense a drop of liquid from the apparatus to a work surface. Insuring contact between liquid dispenser and work surface has typically required complex adjustment means allowing sufficient contact to transfer the desired amount of material while limiting contact to prevent saturation by capillary action.
In addition, various means for measuring small liquid volumes for delivery have been devised. These include electrically-adjusted syringe barrels, manually-filled capillary pipettes, and repetitive filling of a small notch whose liquid contents flow down a delivery arm and collect as a drop on an arrangement of knobs and slits. Use of each of these devices requires contact with a work surface to ensure complete and accurate transfer of the liquid volume.
One embodiment of an invention in the prior art requires no contact with a work surface to accomplish small volume transfer. The embodiment of FIGS. 6 and 7 of U.S. Pat. No. 3,164,304 (H. N. Jager et al.) can transfer by reciprocating motive force a liquid whose volume is determined by a notch in a plunger rod and which collects at the tip of the plunger rod on an arrangement of knobs and slits. The knobs and slits are required to trap the liquid at the tip of the plunger rod. However, accurately detaching the complete, known volume from the knobs and slits without the help of capillary action is difficult.
In recent years, as the sensitivity of analytical techniques has improved dramatically, it has become desirable to analyze samples of smaller and smaller volume. Both manual and automated devices for delivering small fluid volumes have been devised. When dispensing very small volumes using a manual micropipette device, one can visually observe whether a small volume has been completely expelled from the fluid delivery system.
This is not the case, however, for automated systems. To effectively exploit automated robotic sample handling technology, one must be quite confident that all required reagents are consistently and accurately dispensed into the reaction vessel. Also, automated sample handlers must avoid cross-contamination between samples. Cross-contamination results when part of a sample deposited in one receiving vessel is picked up by a pipetting device and deposited into a subsequent vessel. Automated systems can yield inexplicable data and erroneous conclusions if the sample analyzed is impure or if all necessary regents are not present in their required amounts. For that reason, it is important that the tip of the automated fluid delivery system not contact the vessel nor the liquid already deposited in the vessel.
Especially when one processes hundreds or thousands of samples, one is also particularly concerned about the cost per sample and therefore desires to use as little of each reagent as possible while at the same time maintaining concentrations of reactants high enough to sustain the reaction, but not so high or so low as to inhibit the reaction.
What is desired is a micropipette device able to deposit the sub-microliter volumes required by modern analytical techniques, yet flexible enough to accurately deliver many different sub-microliter volumes. In addition a desirable device would deliver such small volumes without contacting a work surface and without risking cross-contamination of samples.
When most or all of a particular liquid has been dispensed, it is necessary to eliminate from the dispensing tip all residual liquid. In existing systems, the dispensing tip is moved to a defined wash or rinse station located at some distance from the stock vessels and receiving vessels. The movement from the dispensing site to the wash or rinse station is inefficient and, in a system making hundreds or even thousands of aspiration and dispensing steps, consumes a significant amount of time that could otherwise be put to more productive use transferring liquids. What is also desired, therefore, is a rinse system that removes residual liquid from the dispensing tip but eliminates the need to position the tip at a dedicated rinse station to do so.